servicemanager: service_manager missing include for string.h

service_manager.c gets string.h inherited from
private/android_filesystem_config.h it should
not rely on this in the future. The intent is
to move fs_config function into libcutils and
thus deprecate any need for string.h in this
include file.

Bug: 19908228
Change-Id: Icc95ee02bf02c596463868b1330d209d1bd5c58a